Non-contact seal for a gas turbine engine

ABSTRACT

A seal comprises the combination of a primary seal and a secondary seal each of which acts on at least one shoe that is installed with clearance relative to one of a rotor and a stator in a position to create a non-contact seal therewith. The at least one shoe is provided with a surface geometry that influences the inertia of fluid flowing across the seal, and, hence, the velocity of the fluid and the pressure distribution across the seal, ultimately affecting the balance of forces applied to the seal.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 11/953,009 filed Dec. 10, 2007, which is acontinuation-in-part of U.S. patent application Ser. No. 11/669,454filed Jan. 31, 2007, which is a continuation-in-part application of U.S.patent application Ser. No. 11/226,836 filed Sep. 14, 2005 and now U.S.Pat. No. 7,182,345, which is a continuation of U.S. patent applicationSer. No. 10/832,053 filed Apr. 26, 2004, now abandoned, which claims thebenefit of U.S. Provisional Application Ser. No. 60/466,979 filed May 1,2003 under 35 U.S.C. § 119(e) for all commonly disclosed subject matter.U.S. Provisional Application Ser. No. 60/466,979 is expresslyincorporated herein by reference in its entirety to form part of thepresent disclosure.

FIELD OF THE INVENTION

This invention relates to seals for sealing a circumferential gapbetween two machine components that are relatively rotatable withrespect to each other, and, more particularly, to a non-contact sealespecially intended for gas turbine engine applications having at leastone shoe supported by a number of spring elements so that a firstsurface of the at least one shoe extends along one of the machinecomponents within design tolerances. The first surface of the at leastone shoe may have a number of different geometries which influence thevelocity and pressure distribution of the fluid flowing across the sealthus allowing the seal clearance to be controlled in both directions,e.g. a larger or smaller radial clearance with respect to a machinecomponent.

BACKGROUND OF THE INVENTION

Turbomachinery, such as gas turbine engines employed in aircraft,currently is dependent on either labyrinth (see FIGS. 1A-1E), brush (seeFIGS. 2A and 2B) or carbon seals for critical applications. Labyrinthseals provide adequate sealing but they are extremely dependent onmaintaining radial tolerances at all points of engine operation. Theradial clearance must take into account factors such as thermalexpansion, shaft motion, tolerance stack-ups, rub tolerance, etc.Minimization of seal clearance is necessary to achieve maximum labyrinthseal effectiveness. In addition to increased leakage if clearances arenot maintained, such as during a high-G maneuver, there is the potentialfor increases in engine vibration. Straight-thru labyrinth seals (FIG.1A) are the most sensitive to clearance changes, with large clearancesresulting in a carryover effect. Stepped labyrinth seals (FIGS. 1B and1C) are very dependent on axial clearances, as well as radialclearances, which limits the number of teeth possible on each land.Pregrooved labyrinth seals (FIG. 1D) are dependent on both axial andradial clearances and must have an axial clearance less than twice theradial clearance to provide better leakage performance than steppedseals.

Other problems associated with labyrinth seals arise from heatgeneration due to knife edge to seal land rub, debris from hardcoatedknife edges or seal lands being carried through engine passages, andexcessive engine vibration. When seal teeth rub against seal lands, itis possible to generate large amounts of heat. This heat may result inreduced material strength and may even cause destruction of the seal ifheat conducted to the rotor causes further interference. It is possibleto reduce heat generation using abradable seal lands, but they must notbe used in situations where rub debris will be carried by leakage airdirectly into critical areas such as bearing compartments or carbon sealrubbing contacts. This also holds true for hardcoats applied to knifeedges to increase rub capability. Other difficulties with hardcoatedknife edges include low cycle fatigue life debits, rub inducedtooth-edge cracking, and the possibility of handling damage. Enginevibration is another factor to be considered when implementing labyrinthseals. As mentioned previously, this vibration can be caused by impropermaintenance of radial clearances. However, it can also be affected bythe spacing of labyrinth seal teeth, which can produce harmonics andresult in high vibratory stresses.

In comparison to labyrinth seals, brush seals can offer very low leakagerates. For example, flow past a single stage brush seal is approximatelyequal to a four knife edge labyrinth seal at the same clearance. Brushseals are also not as dependent on radial clearances as labyrinth seals.Leakage equivalent to approximately a 2 to 3 mil gap is relativelyconstant over a large range of wire-rotor interferences. However, withcurrent technology, all brush seals will eventually wear to line on linecontact at the point of greatest initial interference. Great care mustbe taken to insure that the brush seal backing plate does not contactthe rotor under any circumstances. It is possible for severing of therotor to occur from this type of contact. In addition, undue wire wearmay result in flow increases up to 800% and factors such as changes inextreme interference, temperature and pressure loads, and rubbing speedsmust be taken into account when determining seal life.

The design for common brush seals, as seen in FIGS. 2A and 2B, isusually an assembly of densely packed flexible wires sandwiched betweena front plate and a back plate. The free ends of the wires protrudebeyond the plates and contact a land or runner, with a small radialinterference to form the seal. The wires are angled so that the freeends point in the same direction as the movement of the runner. Brushseals are sized to maintain a tight diametral fit throughout theiruseful life and to accommodate the greatest combination of axialmovement of the brush relative to the rotor.

Brush seals may be used in a wide variety of applications. Althoughbrush seal leakage generally decreases with exposure to repeatedpressure loading, incorporating brush seals where extreme pressureloading occurs may cause a “blow over” condition resulting in permanentdeformation of the seal wires. Brush seals have been used in sealingbearing compartments, however coke on the wires may result inaccelerated wear and their leakage rate is higher than that of carbonseals.

One additional limitation of brush seals is that they are essentiallyunidirectional in operation, i.e., due to the angulation of theindividual wires, such seals must be oriented in the direction ofrotation of the moving element. Rotation of the moving element or rotorin the opposite direction, against the angulation of the wires, canresult in permanent damage and/or failure of the seal. In the particularapplication of the seals required in the engine of a V-22 Ospreyaircraft, for example, it is noted that during the blade fold wing stowoperation, the engine rotates in reverse at very low rpm's. This isrequired to align rotor blades when stowing wings. This procedure isperformed for creating a smaller aircraft footprint onboard an aircraftcarrier. Reverse rotation of the engine would damage or create failureof brush seals such as those depicted in FIGS. 2A and 2B.

Carbon seals are generally used to provide sealing of oil compartmentsand to protect oil systems from hot air and contamination. Their lowleakage rates in comparison to labyrinth or brush seals are well-suitedto this application but they are very sensitive to pressure balances andtolerance stack-ups. Pressure gradients at all operating conditions andespecially at low power and idle conditions must be taken into accountwhen considering the use of carbon seals. Carbon seals must be designedto have a sufficiently thick seal plate and the axial stack load pathmust pass through the plate as straight as possible to prevent coning ofthe seal. Another consideration with carbon seals is the potential forseepage, weepage or trapped oil. Provisions must be made to eliminatethese conditions which may result in oil fire, rotor vibration, andsevere corrosion.

According to the Advanced Subsonic Technology Initiative as presented atthe NASA Lewis Research Center Seals Workshop, development of advancedsealing techniques to replace the current seal technologies describedabove will provide high returns on technology investments. These returnsinclude reducing direct operating costs by up to 5%, reducing enginefuel burn up to 10%, reducing engine oxides of emission by over 50%, andreducing noise by 7 dB. For example, spending only a fraction of thecosts needed to redesign and re-qualify complete compressor or turbinecomponents on advanced seal development can achieve comparableperformance improvements. In fact, engine studies have shown that byapplying advanced seals techniques to just a few locations can result inreduction of 2.5% in SFC.

SUMMARY OF THE INVENTION

This invention is directed to a hybrid, non-contact seal for sealing thecircumferential gap between a first machine component such as a statorand a second machine component such as a rotor which is rotatablerelative to the stator.

In the presently preferred embodiment, the hybrid seal comprises thecombination of a primary seal and a secondary seal each of which acts onat least one shoe extending along one of the rotor and stator in aposition to create a non-contact seal therewith. At least one springelement is connected between one of the rotor and stator and the atleast one shoe. The spring element(s) is flexible in the radialdirection, but axially stiff so that it can function to assist inpreventing roll over of the shoes with respect to the rotor or statorwhere it is located, thus maintaining an effective seal under pressureload. In one embodiment, stops are provided to limit the extent ofradial motion of the shoe with respect to the rotor or stator. Thespring element(s) deflects and moves with the at least one shoe inresponse to the application of fluid pressure applied to the at leastone shoe to create a primary seal, within design tolerances, along thegap between the machine components.

The shoe(s) includes a first, sealing surface and a second surfaceopposite the first surface. The second surface is formed with a slotwithin which one end of a secondary seal may be disposed. It iscontemplated that the slot may be positioned at the front (highpressure) or aft (low pressure) side of the shoe(s). The opposite end ofthe secondary seal is connected to one of the first and second machinecomponents. The secondary seal deflects and moves with the shoe(s) inresponse to the application of fluid pressure applied to the shoe(s),and applies a force acting in the direction of one of the first andsecond machine components to assist with the creation of a secondaryseal along the gap between the machine components.

In the presently preferred embodiment, the first, sealing surface of theshoe(s) may be formed with different geometric features to affect theclearance between the sealing surface of the shoe(s) and the first orsecond machine component. As discussed below, such geometric featuresinfluence fluid inertia, and ultimately the balance of forces applied tothe shoe(s), allowing for improved control of the clearance between theseal and the first or second machine component.

The hybrid seal of this invention can be utilized in all sealapplications, including labyrinth, brush and carbon. The robust designeliminates the careful handling now required of carbon seals utilized inlube system compartments. This seal may allow the engine designer toutilize less parts in the assembly as this seal will permit “blind”assemblies to occur.

The following table provides a comparison of the seal of the subjectinvention with currently available technology.

Dependence Contamination Seal Type Wear Rate Leakage on ClearancesPotential Labyrinth Seals High Low High High Brush Seals Medium LowMedium Medium Carbon Seals Medium Very Low High Low Hybrid Seal Low LowLow Low

DESCRIPTION OF THE DRAWINGS

The structure, operation and advantages of this invention will becomefurther apparent upon consideration of the following description, takenin conjunction with the accompanying drawings, wherein:

FIGS. 1A-1E are schematic views of a number of prior art labyrinthseals;

FIGS. 2A and 2B depict views of a prior art brush seal;

FIG. 3 is an isometric view of the hybrid seal of this invention;

FIG. 4 is a partial, perspective view of the seal depicted in FIG. 3,illustrating a single shoe with the secondary seal removed;

FIG. 5 is a cross sectional view taken generally along line 5-5 of FIG.4;

FIG. 6 is a cross sectional view taken generally along line 6-6 of FIG.3, with a brush seal depicted as a secondary seal;

FIG. 7 is a view similar to FIG. 6 except with a secondary sealcomprising side-by-side plates;

FIG. 8 is an enlarged, side view of a portion of one of the plates shownin FIG. 7;

FIG. 9 is a force balance diagram of a shoe depicting the aerodynamicforces, spring forces and secondary seal forces acting on the shoe; and

FIGS. 10A-10G depict alternative embodiments of shoe(s) having differentgeometric features.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIGS. 3-6, the hybrid seal 10 of this inventionis intended to create a seal of the circumferential gap 11 between tworelatively rotating components, namely, a fixed stator 12 and a rotatingrotor 14. The seal 10 includes at least one, but preferably a number ofcircumferentially spaced shoes 16 which are located in a non-contactposition along the exterior surface of the rotor 14. Each shoe 16 isformed with a sealing surface 20 and a slot 22 extending radiallyinwardly toward the sealing surface 20. For purposes of the presentdiscussion, the term “axial” or “axially spaced” refers to a directionalong the longitudinal axis of the stator 12 and rotor 14, e.g. axis 18shown in FIGS. 3 and 10A-10G, whereas “radial” refers to a directionperpendicular to the longitudinal axis 18.

Under some operating conditions, particularly at higher pressures, it isdesirable to limit the extent of radial movement of the shoes 16 withrespect to the rotor 14 to maintain tolerances, e.g. the spacing betweenthe shoes 16 and the facing surface of the rotor 14. The seal 10preferably includes a number of circumferentially spaced spring elements24, the details of one of which are best seen in FIGS. 3 and 4. Eachspring element 24 is formed with an inner band 26 and an outer band 28radially outwardly spaced from the inner band 26. One end of each of thebands 26 and 28 is mounted to or integrally formed with the stator 12and the opposite end thereof is connected to a first stop 30. The firststop 30 includes a strip 32 which is connected to a shoe 16 (one ofwhich is shown in FIGS. 4 and 5), and has an arm 34 opposite the shoe 16which may be received within a recess 36 formed in the stator 12. Therecess 36 has a shoulder 38 positioned in alignment with the arm 34 ofthe first stop 30.

A second stop 40 is connected to or integrally formed with the strip 32,and, hence connects to the shoe 16. The second stop 40 iscircumferentially spaced from the first stop 30 in a position near thepoint at which the inner and outer bands 26 and 28 connect to the stator12. The second stop 40 is formed with an arm 42 which may be receivedwithin a recess 44 in the stator 12. The recess 44 has a shoulder 46positioned in alignment with the arm 42 of second stop 40.

Particularly when the seal 10 of this invention is used in applicationssuch as gas turbine engines, aerodynamic forces are developed whichapply a fluid pressure to the shoe 16 causing it to move radially withrespect to the rotor 14. The fluid velocity increases as the gap 11between the shoe 16 and rotor 14 increases, thus reducing pressure inthe gap 11 and drawing the shoe 16 radially inwardly toward the rotor14. As the seal gap 11 closes, the velocity decreases and the pressureincreases within the seal gap 11 thus forcing the shoe 16 radiallyoutwardly from the rotor 14. The spring elements 24 deflect and movewith the shoe 16 to create a primary seal of the circumferential gap 11between the rotor 14 and stator 12 within predetermined designtolerances. The purpose of first and second stops 30 and 40 is to limitthe extent of radially inward and outward movement of the shoe 16 withrespect to the rotor 14 for safety and operational limitation. A gap isprovided between the arm 34 of first stop 30 and the shoulder 38, andbetween the arm 42 of second stop 40 and shoulder 46, such that the shoe16 can move radially inwardly relative to the rotor 14. Such inwardmotion is limited by engagement of the arms 34, 42 with shoulders 38 and46, respectively, to prevent the shoe 16 from contacting the rotor 14 orexceeding design tolerances for the gap between the two. The arms 34 and42 also contact the stator 12 in the event the shoe 16 moves radiallyoutwardly relative to the rotor 14, to limit movement of the shoe 16 inthat direction.

In the presently preferred embodiment, the seal 10 is also provided witha secondary seal which may take the form of a brush seal 45, as shown inFIG. 6, or a stack of at least two sealing elements orientedside-by-side and formed of thin sheets of metal or other suitablematerial as shown in FIGS. 7 and 8. The brush seal 45 is positioned sothat one end of its bristles 47 extends into the slot 22 formed in theshoe 16. The bristles 47 deflect with the radial inward and outwardmovement of the shoe 16, in response to the application of fluidpressure as noted above, in such a way as to create a secondary seal ofthe gap 11 between the rotor 14 and stator 12.

Referring now to FIGS. 7 and 8, the secondary seal of this embodimentmay comprise a stack of at least two sealing elements 48 and 50. Each ofthe sealing elements 48 and 50 comprises an outer ring 52 formed with anumber of circumferentially spaced openings 54, a spring member 56mounted within each opening 54 and a number of inner ring segments 58each connected to at least one of the spring members 56. The springmember 56 is depicted in FIG. 8 as a series of connected loops, but itshould be understood that spring member 56 could take essentially anyother form, including parallel bands as in the spring elements 24. Thesealing elements 48 and 50 are oriented side-by-side and positioned sothat the inner ring segments 58 extend into the slot 22 formed in theshoe 16. The spring members 56 deflect with the radial inward andoutward movement of the shoe 16, in response to the application of fluidpressure as noted above, in such a way as to create a secondary seal ofthe gap 11 between the rotor 14 and stator 12. As such, the sealingelements 58 and 50 assist the spring elements 24 in maintaining the shoe16 within design clearances relative to the rotor 14.

In the presently preferred embodiment, the spring elements 48 and 50 areformed of sheet metal or other suitable flexible, heat-resistantmaterial. The sealing elements 48 and 50 may be affixed to one another,such as by welding, a mechanical connection or the like, or they maymerely placed side-by-side within the slot 22 with no connection betweenthem. In order to prevent fluid from passing through the openings 54 inthe outer ring 52 of each sealing element 48 and 50, adjacent sealingelements are arranged so that the outer ring 52 of one sealing element48 covers the openings 54 in the adjacent sealing element 50. Althoughnot required, a front plate 60 may be positioned between the springelement 24 and the sealing element 48, and a back plate 62 may belocated adjacent to the sealing element 50 for the purpose of assistingin supporting the sealing elements 48, 50 in position within the shoe16.

In applications such as gas turbine engines, the seal 10 of thisinvention is subjected to aerodynamic forces as a result of the passageof air along the surface of the shoes 16 and the rotor 14. The operationof seal 10 is dependent, in part, on the affect of these aerodynamicforces tending to lift the shoes 16 radially outwardly relative to thesurface of rotor 14, and the counteracting forces imposed by the springelements 24 and the secondary seals e.g. brush seal 45 or the stackedseal formed by plates 48, 50 which tend to urge the shoes 16 in adirection toward the rotor 14. These forces acting on the shoe 16 areschematically depicted with arrows in FIG. 9. There must be a balance offorces acting on the seal 10 to ensure that nominal clearance ismaintained.

Local pressures acting on the seal 10, induced by the pressuredifferential across the seal 10, have considerable impact on the forcebalance of seal 10. As noted above, when the seal gap 11 increases thefluid velocity increases and the pressure decreases along such gap 11thus drawing the shoe 16 toward the rotor 14. As the seal gap 11 closes,the velocity of the fluid flowing through such gap 11 decreases thusincreasing the pressure and forcing the shoe 16 away from the rotor 16.It has been found that the geometric configuration of the surface of theshoe 10 influences the inertia of fluid flowing across the seal 10, and,hence, the velocity of the fluid and the pressure distribution acrossthe seal 10, ultimately affecting the balance of forces applied to theseal 10. As a result, the radial clearance between the shoes 16 androtor 14 may be either increased or decreased as a result of thegeometric configuration of the surface of shoes 16 that faces the rotor14.

A number of preferred geometries of the shoes 16 are depicted in FIGS.10A-10G. For ease of illustration, only a portion of a shoe 16 isdepicted in FIGS. 10A-10G, and it should be understood that the gap orradial clearance between the shoe 16 and rotor 14 is exaggerated forpurposes of illustration. Generally, each of the shoes 16 shown in FIG.10A-10G include a radially inwardly extending flow contraction area 70,and then variations of converging surfaces, diverging surfaces and othersurfaces, as described individually below. For purposes of discussion ofFIGS. 10A-10D, the term in a “longitudinal direction” refers to adirection along the longitudinal axis 18 of the rotor 14.

Referring to FIG. 10D, the shoe 16 has a first area 72 of substantiallyconstant radial dimension upstream from the flow contraction area 70,and a second area 74 of substantially constant radial dimensiondownstream or aft of the step 70. The radial spacing of the second area74, relative to the rotor 14, is less than that of the first area 72. Aconverging area 76 extends aft from the second area 74, and connects toa diverging area 78. An edge 80 is formed at the juncture of theconverging and diverging areas 76, 78. In the embodiment of FIG. 10A,the length of the converging area 76, measured in a longitudinaldirection along axis 18, is less than the length of the diverging area78.

The shoe 16 illustrated in the embodiment of FIG. 10B has the same flowcontracting area 70, and first and second areas 72, 74, as FIG. 10A. Aconverging area 82 extends from the second area 74 and joins along anedge 84 to a diverging area 86. As seen in FIG. 10B, the length ofconverging area 82, measured along the longitudinal axis 18 of rotor 14,is greater than the length of the diverging area 86.

Referring to FIG. 10C, a shoe 16 is illustrated having the sameconstruction as FIG. 10B, except that instead of a diverging areaconnected to the converging area 86, a third area 88 of substantiallyconstant radial spacing extends from the converging area 86. The radialspacing between the third area 88 and rotor 14 is less than that of thesecond area 74, which, in turn, is less than that of the first area 72.

The converging and diverging areas along the surface of the shoe 16 areeliminated in the embodiment of this invention depicted in FIG. 10D. Thesame first and second areas 72 and 74 connected to step 70 are employed,as described above, but then a second flow contraction area 90 connectsthe second area 74 to an elongated area 91 having a substantiallyconstant radial spacing from the rotor 14. The radial spacing betweenthe elongated area 91 and rotor 14 is less than that of the second area74, which, in turn, is less than that of the first area 72.

The shoe 16 of FIG. 10E is similar to that shown in FIG. 10A, except aconverging area 92 extending from the second area 74, and a divergingarea 94 connected at an edge 96 to the converging area 92, havesubstantially the same length as measured along the longitudinal axis18.

In the embodiment of the shoe 16 illustrated in FIG. 10F, essentiallythe same construction as that depicted in FIG. 10C is provided exceptthe third area 88 is eliminated and a converging area 98 extends fromthe second area 74 to the end of the shoe 16. The same reference numbersused in FIG. 10C are employed in FIG. 10F to indicate common structure.

The shoe 16 of FIG. 10G is similar to that of FIG. 10D, except theelongated area 91 in FIG. 10D is eliminated and replaced with adiverging area 100. The diverging area 100 extends from the second flowcontraction area 90 to the end edge of the shoe 16. All other structureof the shoe 16 shown in FIG. 10G that is common to that of FIG. 10D isgiven the same reference numbers.

While the invention has been described with reference to a preferredembodiment, it should be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out the invention, but that the invention willinclude all embodiments falling within the scope of the appended claims.

1. A seal for sealing a circumferential gap between a first machinecomponent and a second machine component which is rotatable relative tothe first machine component about a longitudinal axis, comprising: atleast one shoe having a first surface and a second surface spaced fromsaid first surface, said first surface extending along one of said firstand second machine components in a position to create a non-contact sealtherewith; at least one spring element connected between one of thefirst and second machine components and said second surface of said atleast one shoe, said at least one spring element being effective todeflect and move with said at least one shoe in response to theapplication of fluid pressure to said at least one shoe in such a way asto assist in the creation of a primary seal of the circumferential gapbetween the first and second machine components; at least one secondaryseal acting on said second surface of said at least one shoe and beingeffective to deflect and move with said at least one shoe in response tothe application of fluid pressure to said at least one shoe in such away as to assist in the creation of a secondary seal of thecircumferential gap between the first and second machine components. 2.The seal of claim 1 in which said first surface is formed with a firstarea of substantially constant radial spacing relative to one of thefirst and second machine components and a contraction area extending ina longitudinal direction from said first area.
 3. The seal of claim 2further including a second area of substantially constant radial spacingextending in a longitudinal direction from said contraction area.
 4. Theseal of claim 3 further including a converging area extending in alongitudinal direction from said second area and in a direction towardone of said first and second machine components.
 5. The seal of claim 4further including a diverging area extending in a longitudinal directionfrom said converging area and in a direction away from said one of saidfirst and second machine components.
 6. The seal of claim 4 furtherincluding a third area of substantially constant radial spacingextending in a longitudinal direction from said converging area.
 7. Theseal of claim 2 further including a second contraction area extending ina longitudinal direction from said second area of substantially constantradial spacing.
 8. The seal of claim 7 further including a third area ofsubstantially constant radial spacing extending in a longitudinaldirection from said second contraction area.
 9. The seal of claim 7further including a diverging area extending in a longitudinal directionfrom said second contraction area and in a direction away from one ofsaid first and second machine components.
 10. A seal for sealing acircumferential gap between a first machine component and a secondmachine component which is rotatable relative to the first machinecomponent about a longitudinal axis, comprising: at least one shoehaving a first surface and a second surface spaced from said firstsurface, said first surface extending along one of said first and secondmachine components in a position to create a non-contact seal therewith,said first surface being formed with a first area of substantiallyconstant radial spacing relative to one of said first and second machinecomponents, a converging area which extends in a direction toward one ofsaid first and second machine components and a flow contraction arealocated between said first area and said converging area; at least onespring element connected between one of the first and second machinecomponents and said second surface of said at least one shoe, said atleast one spring element being effective to deflect and move with saidat least one shoe in response to the application of fluid pressure tosaid at least one shoe in such a way as to assist in the creation of aprimary seal of the circumferential gap between the first and secondmachine components; at least one secondary seal acting on said secondsurface of said at least one shoe and being effective to deflect andmove with said at least one shoe in response to the application of fluidpressure to said at least one shoe in such a way as to assist in thecreation of a secondary seal of the circumferential gap between thefirst and second machine components.
 11. The seal of claim 10 in whichsaid first surface of said at least one shoe further includes a secondarea of substantially constant radial spacing relative to one of saidfirst and second machine components.
 12. The seal of claim 11 in whichsaid substantially constant radial spacing of said second area is lessthan said substantially constant radial spacing of said first area. 13.The seal of claim 11 in which said second area of substantially constantradial spacing is located between said flow contraction area and saidconverging area.
 14. The seal of claim 10 further including a divergingarea extending in a longitudinal direction from said converging area andin a direction away from said one of said first and second machinecomponents.
 15. The seal of claim 10 further including a third area ofsubstantially constant radial spacing relative to one of said first andsecond machine components, said third area extending in a longitudinaldirection from said converging area.
 16. A seal for sealing acircumferential gap between a first machine component and a secondmachine component which is rotatable relative to the first machinecomponent about a longitudinal axis, comprising: at least one shoehaving a first surface and a second surface spaced from said firstsurface, said first surface extending along one of said first and secondmachine components in a position to create a non-contact seal therewith,said first surface being formed with a first area of substantiallyconstant radial spacing relative to one of said first and second machinecomponents, a converging area which extends in a direction toward one ofsaid first and second machine components, a flow contraction are locatedbetween said first area and said converging area, and a diverging areaextending from said converging area in a direction away from said one ofsaid first and second machine components; at least one spring elementconnected between one of the first and second machine components andsaid second surface of said at least one shoe, said at least one springelement being effective to deflect and move with said at least one shoein response to the application of fluid pressure to said at least oneshoe in such a way as to assist in the creation of a primary seal of thecircumferential gap between the first and second machine components; atleast one secondary seal acting on said second surface of said at leastone shoe and being effective to deflect and move with said at least oneshoe in response to the application of fluid pressure to said at leastone shoe in such a way as to assist in the creation of a secondary sealof the circumferential gap between the first and second machinecomponents.
 17. The seal of claim 16 in which said first surface of saidat least one shoe further includes a second area of substantiallyconstant radial spacing relative to one of said first and second machinecomponents, said second area being located between said flow contractionarea and said converging area.
 18. The seal of claim 16 in which each ofsaid converging area and said diverging area of said first surface ofsaid at least one shoe extends in a longitudinal direction with respectto one of said first and second machine components.
 19. The seal ofclaim 18 in which said converging area extends a shorter distance insaid longitudinal direction than said diverging surface.
 20. The seal ofclaim 18 in which said converging area extends substantially the samedistance in said longitudinal direction as said diverging surface. 21.The seal of claim 18 in which said converging area extends a longerdistance in said longitudinal direction than said diverging surface. 22.A seal for sealing a circumferential gap between a first machinecomponent and a second machine component which is rotatable relative tothe first machine component about a longitudinal axis, comprising: atleast one shoe having a first surface and a second surface spaced fromsaid first surface, said first surface extending along one of said firstand second machine components in a position to create a non-contact sealtherewith, said first surface being formed with a first area ofsubstantially constant radial spacing relative to one of said first andsecond machine components, a second area of substantially constantradial spacing relative to one of said first and second machinecomponents, a flow contraction area located between said first area andsaid second area, a third area of substantially constant radial spacingrelative to one of said first and second machine components, and aconverging area located between said second and third areas whichextends in a direction toward one of said first and second machinecomponents; at least one spring element connected between one of thefirst and second machine components and said second surface of said atleast one shoe, said at least one spring element being effective todeflect and move with said at least one shoe in response to theapplication of fluid pressure to said at least one shoe in such a way asto create a primary seal of the circumferential gap between the firstand second machine components; at least one secondary seal acting onsaid second surface of said at least one shoe and being effective todeflect and move with said at least one shoe in response to theapplication of fluid pressure to said at least one shoe in such a way asto assist in the creation of a secondary seal of the circumferential gapbetween the first and second machine components.
 23. The seal of claim22 in which said radial spacing of said second area of said firstsurface of said at least one shoe is less than said radial spacing ofsaid first area thereof.
 24. The seal of claim 22 in which said radialspacing of said third area of said first surface of said at least oneshoe is less than said radial spacing of said second area thereof.
 25. Aseal for sealing a circumferential gap between a first machine componentand a second machine component which is rotatable relative to the firstmachine component about a longitudinal axis, comprising: at least oneshoe having a first surface and a second surface spaced from said firstsurface, said first surface extending along one of said first and secondmachine components in a position to create a non-contact seal therewith,said first surface being formed with a first area of substantiallyconstant radial spacing relative to one of said first and second machinecomponents, a second area of substantially constant radial spacingrelative to one of said first and second machine components, a firstflow contraction area located between said first area and said secondarea, a diverging area extending away from one of said first and secondmachine components, and a second flow contraction area located betweensaid second area and said diverging area; at least one spring elementconnected between one of the first and second machine components andsaid second surface of said at least one shoe, said at least one springelement being effective to deflect and move with said at least one shoein response to the application of fluid pressure to said at least oneshoe in such a way as to assist in the creation of a primary seal of thecircumferential gap between the first and second machine components; atleast one secondary seal acting on said second surface of said at leastone shoe and being effective to deflect and move with said at least oneshoe in response to the application of fluid pressure to said at leastone shoe in such a way as to assist in the creation of a secondary sealof the circumferential gap between the first and second machinecomponents.
 26. The seal of claim 25 in which said radial spacing ofsaid second area of said first surface of said at least one shoe is lessthan said radial spacing of said first area thereof.
 27. A seal forsealing a circumferential gap between a first machine component and asecond machine component which is rotatable relative to the firstmachine component about a longitudinal axis, comprising: at least oneshoe having a first surface and a second surface spaced from said firstsurface, said first surface extending along one of said first and secondmachine components in a position to create a non-contact seal therewith,said first surface being formed with a first area of substantiallyconstant radial spacing relative to one of said first and second machinecomponents, a second area of substantially constant radial spacingrelative to one of said first and second machine components, a firstflow contraction area located between said first area and said secondarea, a third area of substantially constant radial spacing relative toone of said first and second machine components, and a second flowcontraction area located between said second area and said third area;at least one spring element connected between one of the first andsecond machine components and said second surface of said at least oneshoe, said at least one spring element being effective to deflect andmove with said at least one shoe in response to the application of fluidpressure to said at least one shoe in such a way as to assist in thecreation of a primary seal of the circumferential gap between the firstand second machine components; at least one secondary seal acting onsaid second surface of said at least one shoe and being effective todeflect and move with said at least one shoe in response to theapplication of fluid pressure to said at least one shoe in such a way asto assist in the creation of a secondary seal of the circumferential gapbetween the first and second machine components.
 28. The seal of claim27 in which said radial spacing of said second area of said firstsurface of said at least one shoe is less than said radial spacing ofsaid first area thereof.
 29. The seal of claim 27 in which said radialspacing of said third area of said first surface of said at least oneshoe is less than said radial spacing of said second area thereof.